1. Field of the Invention
This invention relates to a power steering apparatus in which the steering force of a steering wheel is assisted by a motor.
2. Description of the Related Art
FIG. 1 is a schematic sectional view illustrating a power steering apparatus which is disclosed in the Japanese patent application laid-open No. 64-41466. In the apparatus, an upper column shaft 2A to which a steering wheel 1 is attached is connected via a torsion bar 4 to a lower column shaft 2B which is coupled through a pinion 5a to a rack 5 of the steering gear. The upper column shaft 2A is rotatably supported by a column housing 3. A torque sensor 6 for detecting a torque applied to the upper column shaft 2A is disposed at the middle portion of the torsion bar 4. The output of the torque sensor 6 is led through a slip ring 11 which is provided in the column housing 3. A known direct drive motor DM of a three-phase motor is disposed at the upper portion of the lower column shaft 2B. The direct drive motor DM includes a rotor 7 fixed to the lower column shaft 2B, and a stator 8 enclosing the rotor 7 and having a coil 8a. The stator 8 is fixed to the inner wall of a motor case 9 which is attached to the lower end of the column housing 3.
FIG. 2 is a circuit diagram illustrating a conduction control circuit of the direct drive motor DM. A series circuit of power MOSFETs (hereinafter, referred to as "power transistors") T1 and T4 is connected between a DC power source +V and the ground E. To this series circuit, two series circuits respectively consisting of power transistors T2 and T5 and power transistors T3 and T6 are connected in parallel. The junction point A of the power transistors T1 and T4 is coupled to one end of the U-phase coil 8au of the coil 8a in the direct drive motor DM, the junction point B of the power transistors T2 and T5 to one end of the V-phase coil 8av, and the junction point C of the power transistors T3 and T6 to one end of the W-phase coil 8aw. The other ends of the coils 8au, 8av and 8aw are connected in common. A signal for commutation-controlling the power transistors T1-T6 is supplied from a control unit (not shown) to the gates G1-G6 of the power transistors T1-T6.
The operation of this power steering apparatus will be described.
When the steering wheel 1 is turned, a torque is applied to the upper column shaft 2A, and is detected by the torque sensor 6. A torque signal corresponding to the torque which is detected by the torque sensor 6 is inputted to the control unit (not shown). In accordance with this torque signal, the control unit supplies a signal, for commutation-controlling the power transistors T1-T6, to the gates G1-G6, thereby commutation-controlling the power transistors T1-T6. The currents flowing through the phase coils 8au, 8av, and 8aw are controlled by adjusting the conduction angles of the power transistors T1-T6, so as to control the rotating force of the direct drive motor DM. By changing the sequence of commutation-controlling the power transistors T1-T6, the direction of rotation of the direct drive motor DM is controlled. The rotation of the thus-driven direct drive motor DM causes the lower column shaft 2B to rotate, thereby assisting the steering force.
This assistance to the steering force decreases the torque applied to the upper column shaft 2A. When the torque signal ST from the torque sensor 6 is lowered to the dead zone, it becomes unnecessary to assist the steering force, and therefore the direct drive motor DM is stopped. In this case, even when the current supply to the phase coils 8au, 8av and 8aw is shut off, the rotor 7 still rotates owing to inertia. Therefore, the power transistors T1, T2, and T3 are turned off and the power transistors T4, T5, and T6 are turned on, so that the phase coils 8au, 8av and 8aw are short-circuited as shown in FIG. 3, and the rotor 7 is braked to immediately stop the rotation by the short circuit current.
After the phase coils are short-circuited, however, a counter electromotive force is generated in each of the phase coils 8au, 8av , and 8aw during a short period of time until the rotor stops, whereby a reversal rotating force acts on the rotor which stops, whereby a reversal rotating force acts on the rotor which applies an opposite rotating force to the steering wheel to that by the above-mentioned assisting force. This causes a feeling of physical disorder in the steerage and the return of the steering wheel to the straight cruising position is not well done.